Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Increased concern regarding the consumption of fossil fuels and reduction of greenhouse gases has led to research and development focusing on maximising the efficiency in power generation and on renewable energy resources. One renewable energy resource is geothermal energy, which is derived from the thermal energy stored deep within the Earth. Whilst increasing the efficiency of power generation is a common concern for all energy resources, it is of particular interest for geothermal power plants.
The production of power from geothermal energy basically involves extracting geothermal fluid from a reservoir and converting the thermal energy stored in the geothermal fluid into mechanical work and then into electricity. Conventional geothermal power cycles can generally be classified into non-condensing direct steam cycles, condensing direct steam cycles (single flash or double flash), binary cycles and combined cycles. All but condensing direct steam cycles use a working fluid to exchange heat with the geothermal heat source and drive the turbine to generate power. Condensing direct steam cycles are limited to dry-steam geothermal reservoirs, which are much rarer than other geothermal reservoirs, such as hot-water and hot-dry-rock reservoirs.
However, these conventional power cycles were originally designed for large scale power production from fossil fuels, where higher temperature sources are available for heat exchange. Consequently, in these conventional power cycles, the evaporation and condensation of the working fluid both occur at constant temperatures. In the context of geothermal sources, this results in large temperature mismatches between the working fluid and the geothermal heat source during the heat addition or the rejection processes in the thermodynamic cycle. For example, in a binary cycle, the temperature difference between the working and geothermal fluids in the primary heat exchanger could be as high as 80° C. to 100° C. In terms of thermodynamics, larger temperature differences in the heat exchange process increase the entropy in the power cycle, thereby reducing the efficiency, particularly the second law efficiencies related to exergy (availability), of the heat exchange process and resulting in poor energy recovery for power generation.
To address this problem, the Kalina cycle employs a multi-component zeotropic mixture of ammonia and water as its working fluid, and additional absorption and distillation equipment to reconstitute the mixture at the low temperature end of the cycle. The multi-component working fluid has a variable phase change temperature during evaporation so that the evaporation of the working fluid occurs over a range of temperatures. Hence, the mixture temperature can more closely match the temperature of the geothermal fluid to increase the amount of thermal energy that is recovered and minimising the entropy in the cycle, thus improving the efficiency of the heat exchange process for low temperature applications, such as geothermal heat sources, as opposed to fossil fuel based power generation.
A disadvantage of the Kalina cycle is that the absorption and distillation equipment added to the cycle creates further complexity to the system, and significantly increases the cost of plant installation compared with other types of power plants. Furthermore, the Kalina cycle has a high sensitivity towards the pressure and composition of the ammonia-water mixture, which limits the operation of the cycle over the whole range of possible geothermal reservoir temperatures and effectively sets a lower limit to the minimum temperature at which a deep geothermal energy source may be commercialised.